A federal judge in California ruled today that the state's same-sex marriage ban amounts to unconstitutional discrimination and should be immediately struck down.
"Proposition 8 fails to advance any rational basis in singling out gay men and lesbians for denial of a marriage license," wrote U.S. District Court Judge Vaughn Walker in a 136-page decision. "Indeed, the evidence shows Proposition 8 does nothing more than enshrine in the California Constitution the notion that opposite-sex couples are superior to same-sex couples."
Walker issued a stay on the order overturning Prop 8 to allow supporters of the measure to argue why it should remain in effect while pursuing an appeal. The case is expected to reach the U.S. Supreme Court.
The controversial ballot measure was launched in response to a state supreme court decision allowing gays and lesbians to wed. The ballot measure passed with 52 percent of the vote in November 2008.
"With a stroke of his pen, Judge Walker has overruled the votes and values of 7 million Californians who voted for marriage as one man and one woman," said Brian Brown, president of the National Organization for Marriage, in a statement. "This ruling, if allowed to stand, threatens not only Prop 8 in California but the laws in 45 other states that define marriage as one man and one woman."
Lawyers supporting the ballot measure had argued that voters endorsed a "fundamental, definitional feature" of marriage that has historical roots "in this country and, almost without exception, in every civilized society that has ever existed."
But Walker found the plaintiffs in the case -- one lesbian and one gay couple -- demonstrated by "overwhelming evidence" that Proposition 8 violates their rights to due process and equal protection under the Constitution.
"Moral disapproval alone is an improper basis on which to deny rights to gay men and lesbians. The evidence shows conclusively that Proposition 8 enacts, without reason, a private moral view that same-sex couples are inferior to opposite-sex couples," wrote Judge Walker.
"We are thrilled with today's ruling," said Geoff Kors, executive director for Equality California, which filed an amicus brief supporting the Prop. 8 federal challenge. "Judge Walker has preserved our democracy by ruling that a majority cannot deny a minority group of fundamental freedoms."
The verdict will now be appealed to the Ninth Circuit Court of Appeals and may ultimately reach the U.S. Supreme Court, where experts say a decision could transform social and legal precedent. They compare the potential impact to the famous 1954 Brown v. Board of Education decision, which desegregated schools, and the 1967 Loving v. Virginia decision, which ended laws banning interracial marriage.
Outside the courthouse a crowd of mainly gay couples broke into celebration after word of Walker's ruling. Some shed tears while others started saying their wedding vows ceremoniously.
A group even marched to San Francisco City Hall to get legally married, but when they arrived there they learned about the stay that had been issued.
"I think something that the best-intentioned backers of Prop 8 don't understand is that it basically hurts," said Jordan Krueger, 29, of Los Angeles, who is engaged to his partner Hank, 40.
"It has been a difficult struggle, going back and forth, always in limbo," he said. "But whether or not I can be married to my partner really has no impact on others' marriages and that's what the court said today."
The California case, which involved a 13-day trial that concluded in June, was the first in federal court to challenge a state prohibition on same-sex marriage.
Brown of the National Organization for Marriage, which has helped finance the legal case supporting Prop 8, said he expected the decision, and believes it will revive a nationwide debate.
"Judge Walker just fired the first salvo in a major culture war, and the latest example of a court crossing the line... People won't stand for marriage being redefined," he said.
Attorney Andy Pugno, of the conservative advocacy group Protect Marriage, and former Justice Department lawyer Charles Cooper have been defending Proposition 8, after neither California Attorney General Jerry Brown nor Gov. Arnold Schwarzenegger moved to defend the measure on behalf of the state.
Representing the plaintiffs is a pair of unusual bedfellows: former Republican Solicitor General Ted Olson and Democrat David Boies. The two prominent attorneys opposed each other before the Supreme Court in the case of Bush v. Gore, which decided the persidential election in 2000.
A California Field poll of registered voters last month found 51 percent support legalizing gay marriage with 42 percent opposed.
Nationwide public views are more narrowly divided, according to the most recent ABC News/Washington Post poll. Forty-seven percent of Americans polled favor gay marriage while 50 percent are opposed.
Five states -- Massachusetts, Iowa, Connecticut, Vermont and New Hampshire -- and the District of Columbia currently perform same-sex marriages. Four states recognize marriages performed elsewhere, and nine states grant civil unions or partnerships.
ABC News' Alex Stone contributed to this report.